The steam generators of pressurized-water nuclear power stations generally comprise a bundle consisting of a very large number of tubes of small diameter which are curved in a U and fixed at each of their ends by crimping in a thick tube plate. That part of the steam generator situated below the tube plate constitutes a water reservoir within a first part of which the pressurized "primary" water coming from the vessel of the reactor is distributed within the tubes of the bundle. In a second part, the water which has circulated in the tubes is recovered, in order to be sent back, through a conduit of the primary circuit of the reactor, into the vessel enclosing the core formed by the fuel assemblies ensuring the heating of the pressurised water. The feed water of the steam generator comes into contact with the external surface of the tubes of the bundle, in that part of the steam generator situated above the tube plate, in order to be converted there into steam, and this steam is sent to the turbine associated with the nuclear reactor.
The walls of the tubes of the bundle thus constituted a barrier between the pressurised water constituting the primary fluid of the reactor and the feed water constituting the secondary fluid. The primary fluid which comes into contact with the fuel assemblies and the internal structures of the vessel of the reactor contains radioactive products. It is thus appropriate to avoid placing of the primary fluid in contact with the secondary fluid, the contamination of which might have very undesirable consequences. To this end, it is necessary to avoid to the greatest possible extent the appearance of leaks through the walls of the tubes of the bundle of the steam generator and to repair as quickly as possible and in an effective manner any tubes, the wall of which is pierced.
Such repairs are generally necessary during the normal service life of the reactor. In fact, as a result of the stresses of thermal and mechanical origin or of the corrosion suffered by the tubes of the bundle, cracks may be formed in the walls of the tubes of the bundle, both on the primary side and on the secondary side. These repairs are carried out during maintenance of the nuclear reactor.
A process is known for repairing the tubes of the bundle of a steam generator by sleeve coupling of these tubes at the location of the defect involving a loss of sealing. A sleeve having an external diameter slightly less than the internal diameter of the tube to be repaired and having a length which is sufficient to cover the defective zone is introduced into the tube, by the entrance face of the tube plate on which the ends of the tubes are aligned. It is placed thereat, in such a manner as to be aligned with the entrance face of the tube plate for the tubes situated in the central part of the latter, or at mid-thickness of the tube plate for the tubes situated at the periphery of the latter. It is then fixed within the tube in such a manner as to effect two sealed joints between itself and the tube on either side of the defective zone.
Numerous methods are known for effecting the fixing of the sleeve on either side of the defective zone. In particular, it is known to effect diametric expansion of the sleeve in two zones close to its ends situated, in the case of one of them, within the thickness of the tube plate and, in the case of the other, beyond the exit face of the tube plate, this taking place after cleaning of the tube in these two zones. The expansion may be obtained either by hydraulic or by mechanical methods, such as rolling in, or by expansion by explosion, or by welding or brazing.
All these operations necessitate the taking of action within the water reservoir of the steam generator, i.e., within a part of this generator which has been in contact with the primary fluid loaded with radioactive products, during the operation of the reactor, and where a human operator is subjected to a high degree of radioactivity.
Automatic and remotely controlled devices have thus been proposed, which are capable of effecting at least partially the operations required for the sleeve coupling of the tubes.
There are known, for example, a process and a device permitting the installation, within a leaky generator tube, of a sleeve from the exterior of the water reservoir. This process and this device have formed the subject of the patent application FR 2,598,209. The sleeve is introduced into the tube from the exterior of the water is introduced into the tube from the exterior of the water reservoir by a flexible tube referred to as a boa, on a double hydraulic expander with which the approach within the tube is performed: after a sufficient expansion effected in the two zones situated at the ends of the sleeve by inflation of the elastic membranes of the double expander, the sleeve remains positioned within the tube.
The membranes are then decompressed and the expander is withdrawn from the sleeve. The fixing tools, top tube expander and bottom tube expander, are then successively introduced by the same flexible component into the sleeve, respectively into the top zone and then into the bottom zone, in order to effect the fixing of the sleeve by an operation of rolling in.
The disadvantage of this method is that it does not permit the production of rigorously sealed fixings of the sleeve.
In order to remedy this disadvantage, it is known to effect the fixing by laser welding of the sleeve in the tube at each end of the sleeve.
Applicant's application FR 2,637,829 proposes a tooling of the type indicated above, in which the laser is of the YAG type and its beam is conveyed by an optical fiber.